The invention relates to an integrated circuit arrangement with at least one transistor and to a method for its production.
For an integrated circuit arrangement, that is to say an electronic circuit which is integrated in a substrate, a high packing density is advantageous since, firstly, its switching speed is high because of short distances between its components and, secondly, its dimensions are low.
L. Risch et al, Vertical MOS Transistors with 70 nm channel length, ESSDERC (1995) 101, describes a transistor whose source/drain regions and channel region are arranged under one another. This so-called vertical transistor takes up less area than a conventional planar transistor whose source/drain regions and channel region are arranged beside one another and, consequently, can contribute to increasing the packing density of an integrated circuit arrangement. It is feared, however, that in the case of this transistor, floating-body effects occur, such as leakage currents on account of a parasitic bipolar transistor. In particular, at high frequencies, it is probable that the channel region will be electrically charged.
H. Takato et al, xe2x80x9cHigh Performance CMOS Surrounding Gate Transistor (SGT) for Ultra High Density LSIsxe2x80x9d, IEDM (1988) 222, describes a vertical transistor in which a lower source/drain region is not arranged directly under a channel region but underneath but offset laterally in relation to the former. The channel region is electrically connected to the substrate. In order to produce the transistor, a silicon island is etched into a substrate. A gate dielectric and a spacer-like gate electrode, which surrounds the silicon island at the side, are then produced. By means of implantation, the upper source/drain region is produced in an upper part of the silicon island, and the lower source/drain region is produced outside and laterally adjacent to the silicon island. The channel region is arranged in the silicon island, underneath the upper source/drain region. Consequently, the channel length is determined by the etching depth during the production of the silicon island.
German patent 195 19 160 C1 has proposed a DRAM cell arrangement in which each memory cell comprises a projection-like semiconductor structure which comprises a first source/drain region, a channel region arranged underneath, and a second source/drain region arranged under the latter, and which is surrounded annularly by a gate electrode. Semiconductor structures of memory cells are arranged in rows and columns. In order to produce word lines in a self-adjusting manner, that is to say without the use of masks which have to be adjusted, spacings between semiconductor structures arranged along the columns are smaller than spacings between semiconductor structures arranged along the rows. The word lines are produced by depositing and etching back conductive material in the form of gate electrodes adjoining one another along the columns.
The invention is based on the object of specifying an integrated circuit arrangement having at least one transistor in which floating-body effects in the transistor can be avoided and which, at the same time, can be produced with an increased packing density and process accuracy in comparison with the prior art. In addition, a method of producing such a circuit arrangement is specified.
The problem is solved by an integrated circuit arrangement having at least one vertical MOS transistor, for which a substrate is provided which, in a layer adjacent to a surface of the substrate, is doped with a first conductivity type. Arranged on the substrate is a structured sequence of layers having a lower layer, a central layer doped with the first conductivity type and an upper layer. The sequence of layers has at least one first lateral and a second lateral face, which are each formed by the lower layer, the central layer and the upper layer. The lower layer can be used as a first source/drain region of the transistor, the central layer can be used as a channel region of the transistor, and the upper layer can be used as a second source/drain region of the transistor. In order to connect the channel region electrically to the substrate, a connecting structure doped with the first conductivity type is arranged on at least the first face of the sequence of layers in such a way that it laterally adjoins at least the central layer and the lower layer and reaches into the substrate. A gate dielectric adjoins at least the second face of the sequence of layers, and a gate electrode of the transistor adjoins the gate dielectric.
The problem is further solved by a method of producing an integrated circuit arrangement having at least one vertical MOS transistor in which, in order to form a sequence of layers on a substrate which is doped with a first conductivity type in a layer adjacent to a surface of the substrate, firstly a lower doped layer is produced, which can be used as a first source/drain region of the transistor, above this a central layer doped with the first conductivity type is produced, which can be used as the channel region of the transistor, and above that a doped upper layer is produced, which can be used as a second source/drain region of the transistor. In order to connect the channel region electrically to the substrate, a connecting structure doped with the first conductivity type is produced on a first face of the sequence of layers in such a way that it laterally adjoins at least the central layer and the lower layer and reaches into the substrate. The sequence of layers is structured in such a way that a second face of the sequence of layers is produced opposite the first face. A gate dielectric and, adjacent thereto, a gate electrode are produced at least on the second face of the sequence of layers.
The channel length of the transistor of the circuit arrangement is determined by the thickness of the central layer. As compared with the transistor according to H. Takato et al (see above), in which the channel length is determined by an etching depth, the channel length can be set more accurately. Consequently, the circuit arrangement can be produced with an increased process accuracy.
The connecting structure permits charge to flow away from the channel region, so that, as opposed to the transistor according to Risch et al (see above), floating-body effects are avoided. The channel region is not charged up electrically, even at high frequencies.
In order to avoid leakage currents, the connecting structure preferably consists of monocrystalline semiconductor material, such as silicon and/or germanium. The connecting structure is produced, for example, by epitaxy in a trench which cuts into or cuts through the sequence of layers. It is advantageous to provide a low dopant concentration, for example up to 3*1017 cmxe2x88x923, of the connecting structure, in order to keep capacitances between the substrate and the gate electrode small.
Alternatively, polycrystalline semiconductor material, such as polysilicon, can be used for the connecting structure. In this case, the trench is filled with the semiconductor material. Alternatively, the semiconductor material can be applied in a thickness which is not sufficient to fill the trench. The semiconductor material can then be etched back, so that the connecting structure is produced in the form of a spacer. If the connecting structure comprises polycrystalline material or material with a large number of defects, it is advantageous to provide a high dopant concentration of the connecting structure, for example 5*1018 cmxe2x88x923 to 1020 cmxe2x88x923, in order to reduce the expansion of space charge zones into the connecting structure.
In order to increase breakdown voltages between the connecting structure and the source/drain regions and, at the same time, to prevent space charge zones reaching through, the scope of the invention includes increasing the dopant concentration of the connecting structures during their production, so that inner parts of the connecting structures are more highly doped than outer parts.
A particularly high packing density of the circuit arrangement may be achieved if a width of the connecting structure and/or a spacing between the first face and the second face of the sequence of layers, which lies opposite the first face, are smaller than the minimum structure size F that can be produced in the photolithography applied for the production of the circuit arrangement.
In order to produce such a narrow sequence of layers, a spacer can be used as a mask.
Since the connecting structure is produced on the first face of the sequence of layers, and the gate dielectric is produced on the second face of the sequence of layers, and therefore the two faces are subjected to different process steps, it is advantageous if the sequence of layers is produced in two different process steps. For this purpose, a mask is applied to the surface, said mask leaving at least an area of F2, of the upper layer free. The mask is widened by a spacer in that material is deposited and etched back. As a result, the exposed area of the upper layer is reduced to sub-lithographic dimensions. In order to produce the trench and therefore the first face of the sequence of layers, the exposed area of the upper layer is subjected to a first etching process, etching being carried out selectively in relation to the spacer and the mask. The connecting structure is then produced. The mask is removed selectively in relation to the spacer. By means of a second etching process, the second face of the sequence of layers is produced, etching being carried out selectively in relation to the spacer.
If the connecting structure consists of the same semiconductor material as the output layer, the central layer or the lower layer, then an auxiliary structure is produced above the connecting structure, in order to protect the connecting structure during the production of the second face of the sequence of layers. If an upper face of the connecting structure is located under an upper face of the mask, then the auxiliary structure can be produced by a material being deposited and planarized until the mask is exposed.
The circuit arrangement can have a further sequence of layers, constructed analogously to the sequence of layers, whose first face adjoins the connecting structure in such a way that the connecting structure is arranged between the sequence of layers and the further sequence of layers, and a central layer of the further sequence of layers is electrically connected to the substrate. A further gate dielectric adjoins at least a second face of the further sequence of layers, and a further gate electrode adjoins the further gate dielectric.
In order to simplify the process, the sequence of layers and the further sequence of layers are preferably produced by structuring a single upper layer, central layer and lower layer. Alternatively, the sequences of layers are produced, for example, by selective epitaxy within a suitable mask.
In the following text, the description xe2x80x9cupper layerxe2x80x9d is used for the continuous upper layer which is produced at the beginning of the production method and from which parts of the sequence of layers are produced. This is similarly true of the xe2x80x9ccentral layerxe2x80x9d and the xe2x80x9clower layerxe2x80x9d. On the other hand, xe2x80x9cthe upper layer of the sequence of layersxe2x80x9d describes only a specific part of this sequence of layers. If the sequences of layers are produced from the upper layer, then xe2x80x9cthe upper layer of the sequence of layersxe2x80x9d means the same as xe2x80x9cthat part of the upper layer belonging to the sequence of layersxe2x80x9d.
If the upper layer of the sequence of layers and an upper layer of the further sequence of layers are used as the second source/drain region of the transistor, the central layer of the sequence of layers and the central layer of the further sequence of layers are used as the channel region of the transistor, and the lower layer of the sequence of layers and a lower layer of the further sequence of layers are used as the first source/drain region of the transistor, then the transistor has a particularly large channel width. The gate electrode and the further gate electrode form a common gate electrode.
The upper layer, the central layer and the lower layer can be structured in such a way that they surround the connecting structure, so that the sequence of layers and the further sequence of layers merge into each other. The sequence of layers and the further sequence of layers can alternatively be separated by the connecting structure. In the first case, the mask leaves an area, for example a square area, free, so that an intrinsically closed spacer is formed during the widening of the mask, and therefore a correspondingly structured sequence of layers can be produced. In the second case, the mask is strip-like, for example, so that two spacers separated from each other, and therefore two sequences of layers separated from each other, are produced.
A region doped with a second conductivity type opposite to the first conductivity type can be arranged above the connecting structure, in order to connect the upper layer of the sequence of layers and the upper layer of the further sequence of layers electrically to each other.
In order to produce the doped region, an upper part of the connecting structure can be implanted, so that the aforesaid upper part is converted into the doped region.
If the upper layer of the further sequence of layers is used as a second source/drain region of a further transistor, the central layer of the further sequence of layers is used as a channel region of the further transistor, and the lower layer of the further sequence of layers is used as a first source/drain region of the further transistor, then the packing density of the circuit arrangement is particularly high, since the connecting structure acts, firstly, as a common connecting structure of the two sequences of layers and, secondly, separates the transistors from each other. Here, too, the doped region can be provided, so that the two transistors are connected in series.
The circuit arrangement can be used, for example, as a memory cell arrangement. The sequence of layers and the further sequence of layers form a pair, the second face of the sequence of layers being opposite the first face of the sequence of layers, and the first face of the further sequence of layers being opposite the first face of the sequence of layers. A number of pairs analogous to the pair are arranged in an xy grid. At least some of the pairs are separated from each another by first dividing trenches running substantially parallel to one another, so that alternately one of the pairs and one of the first dividing trenches are arranged beside each other, and the second faces of the sequences of layers of the pairs adjoin the first dividing trenches. Word lines, which run transversely with respect to the first dividing trenches, are connected to the gate electrode. Lower bit lines are preferably parts of the lower layer and run transversely with respect to the word lines. The trenches in which the connecting structures belonging to the pairs are produced are produced in the form of strips. The first dividing trenches run parallel to the trenches. The lower layer is structured at least by the trenches, so that the lower bit lines adjoin the connecting structures and run parallel to them.
If the first dividing trenches are produced in such a way that they reach down into the lower layer without cutting through them, alternately one of the lower bit lines and one of the trenches are arranged beside each other. Such a circuit arrangement can be used, for example, as a ROM cell arrangement. Pairs which are arranged between two of the first dividing trenches which are adjacent to each other merge into each other, so that the associated connecting structures form a common connecting structure which has cross sections parallel to the surface which are strip-like and run substantially parallel to the first dividing trenches. This applies similarly to the doped regions which form upper bit lines. The gate electrodes are parts of the word lines which have strip-like cross sections parallel to the surface. Each pair is part of two transistors which are connected in series and which are each connected between one of the upper bit lines and one of the lower bit lines. A memory cell comprises one transistor. The memory cell can be produced with an area of 2F2. Information is stored in the form of dopant concentrations of the channel regions and therefore in the form of threshold voltages of the transistors. In order to read the information out of a transistor, the associated word line is activated and a measurement is made to see whether or not a current flows between the associated upper bit line and the associated lower bit line. The dopant concentrations of the channel regions of the transistors can be adjusted by means of masked, oblique implantation.
If the first dividing trenches are produced in such a way that they cut through the lower layer, then one of the lower bit lines is arranged between one of the trenches and one of the first dividing trenches. Such a circuit arrangement can be used, for example, as a DRAM cell arrangement. It is also advantageous in this memory-cell arrangement if connecting structures of pairs which are arranged between two mutually adjacent first dividing trenches form a common connecting structure. The connecting structure has a cross section which is parallel to the surface, is strip-like and runs parallel to the first dividing trenches. In the case of the DRAM cell arrangement, however, the pairs which are arranged between two of the first dividing trenches which are adjacent to each other do not merge into each other but are separated from each other by second dividing trenches. The second dividing trenches run transversely with respect to the first dividing trenches and reach down into the lower layer. The second dividing trenches do not cut through the lower layer, in order that the lower bit lines are not interrupted. In addition, the connecting structures are not interrupted by the second dividing trenches.
It is within the scope of the invention if in each case one of the pairs is part of one of the transistors. In this case, it is advantageous to provide the doped regions which connect the upper layers of the sequences of layers of the pair to each other. The transistor is connected to two lower bit lines, which adjoin the associated connecting structure. The two lower bit lines are connected together, for example in a periphery of the DRAM cell arrangement, and act as a single bit line.
It is within the scope of the invention if the upper layers and the doped regions act as first capacitor electrodes of capacitors. For this purpose, a capacitor dielectric is arranged above the upper layers and the doped regions and, above this, a second capacitor electrode, which can be designed as a common capacitor plate of all the capacitors.
As opposed to the ROM cell arrangement, in which the word lines run above the upper layers, the word lines of the DRAM cell arrangement are preferably configured in a different way, since they otherwise run above the capacitors which are arranged above the upper layers. The word lines are, for example, formed by the gate electrodes, which surround the pairs annularly at the sides and adjoin one another within the first dividing trenches.
A memory cell of the DRAM cell arrangement comprises one of the transistors and one of the capacitors, which are connected in series with each other. The memory cell can be produced with an area of 4F2.
The information in a memory cell is stored in the form of a charge on the associated capacitor.
In order to increase the packing density, it is within the scope of the invention if in each case one of the pairs is part of two transistors. In this case, the doped regions are not provided, in order that the transistors are separated from each other. The lower bit lines act as individual bit lines. A memory cell in such a DRAM cell arrangement can have an area of only 2F2.
A FRAM (ferroelectric RAM) cell arrangement is produced if the capacitor dielectric contains a ferroelectric material.
The mask for producing the sequences of layers and the connecting structures can comprise a first auxiliary layer and a second auxiliary layer arranged above the first, it being possible for the first auxiliary layer to be etched selectively in relation to the spacer, and for the second auxiliary layer to be etched selectively in relation to the semiconductor material.
In order to produce the memory cell arrangements, the first auxiliary layer and the second auxiliary layer are structured in a strip-like manner, so that the upper layer is partially exposed. During the production of the trenches, the second auxiliary layer and the spacers act as a mask. During the production of the auxiliary structures, the second auxiliary layer and the spacers are removed until the first auxiliary layer is exposed. The first auxiliary layer is then removed selectively in relation to the spacers and auxiliary structures, so that the spacers and the auxiliary structures can act as a mask during the production of the first dividing trenches.
In order to protect the upper layer, a protective layer can be produced between the upper layer and the first auxiliary layer. The protective layer, the second auxiliary layer, the spacers and the auxiliary structures can contain SiO2, for example. The first auxiliary layer can contain polysilicon, for example.
The word lines of the DRAM cell arrangement can be produced in a self-adjusting is manner, that is to say without the use of masks to be adjusted, if spacings between pairs which are adjacent to each other transversely with respect to the first dividing trenches are smaller than spacings between pairs which are adjacent to each other parallel to the first dividing trenches. In this case, in order to produce the word lines, material can be deposited in such a thickness that the first dividing trenches are filled but not the second dividing trenches. By means of etching back, spacers are then produced in the second dividing trenches, while bottoms of the first dividing trenches continue to remain caboveed by material. As a result, therefore, the gate electrodes are produced without masks, surround the pairs annularly and adjoin one another within the first trenches.
In order to implement the spacings of different sizes at a high packing density, following the removal of the first auxiliary layer, further spacers adjacent to the spacers can be produced by material being deposited and etched back. The further spacers preferably consist of the same material as the spacers. The first dividing trenches are produced, the spacers, the further spacers and the auxiliary structures acting as a mask. The first dividing trenches produced in this way are narrower than the first dividing trenches of the above-described ROM cell arrangement. The second dividing trenches can be produced with a lithographically structured mask, so that their widths are considerably greater than the widths of the first dividing trenches and, for example, are of a size F.
In the following text, exemplary embodiments of the invention, which are illustrated in the figures, will be explained in more detail.